The present invention is related generally to the field of positional tracking using an electromagnetic field and, more particularly, to such tracking using a rotating dipole field.
The prior art contains many examples of the use of an electromagnetic field for tracking and position monitoring purposes. A number of these prior art implementations specifically utilize an electromagnetic dipole field. In particular applications, such as, for example, horizontal directional drilling, a dipole field is particularly useful, at least in part, as a result of its axisymmetric field pattern. That is, the axis of the dipole field can be oriented along the axis of a rotating boring tool in order to create a quasistatic field. It is noted that the electromagnetic dipole field is advantageous with respect to the ability to modulate the field with information of interest.
A more limited number of prior art implementations make use of a dipole field wherein the dipole field axis is rotated about an axis that is generally perpendicular to the dipole field axis for purposes of monitoring position and/or orientation. Such a field can be created in the form of an electromagnetic field by passing electrical current through coils. One use of a rotating electromagnetic dipole field is described by U.S. Pat. No. 3,121,228, issued to Kalmus (hereinafter the '228 patent) and entitled DIRECTION INDICATOR. This patent teaches a system which transmits a rotating electromagnetic dipole field from a leading vehicle which is received by a following vehicle. The field generator on the leading vehicle and the receiver on the following vehicle are each made up of a pair of orthogonal coils. The field generator is driven by an alternating current source with a 90 degree phase difference between its coils. A phase difference between the receiver coils is used to determine orientation, while the distance between the receiver and generator is determined based on the sum of the induced voltages in the receiver coils. It is of interest, however, that the solution which is presented assumes that the signal generator and the receiver are coplanar with respect to one another, which is typically only the case with a level ground surface. It is considered that this constraint markedly narrows the usefulness of the described method.
Another early prior art system is described in U.S. Pat. No. 4,812,812. Unfortunately, this system does not appear to be capable of producing distance measurements. Further, it is submitted that the system is inoperable in the event that the transmitter is higher than the receiver.
Another prior art implementation which uses a rotating dipole field is described in U.S. Pat. No. 5,589,775 issued to Kuckes. In this implementation, the rotated dipole field is applied to the purpose of forming a horizontal borehole in a parallel spaced apart relationship with a pre-existing borehole. Formation of such a parallel borehole is useful, for example, in steam-assisted gravity drainage (SAGD) of heavy oils. Like the '228 patent, Kuckes provides a solution in which the receiver and field generator are assumed to be coplanar, as described in column 9 of the Kuckes disclosure. Any deviation from this coplanar relationship is described as being so small as to be insignificant. That is, the Kuckes patent embodies the same limiting constraint that is embodied by the '228 patent by ignoring out-of-plane displacements.
U.S. Pat. Nos. 5,646,524 and 5,646,525 issued to Gilboa (hereinafter, the Gilboa patents) are directed to a targeting system for detecting the position and orientation of a helmet using a rotating magnetic field. Unfortunately, it appears that the techniques introduced by the Gilboa patents are limited at least with respect to their use of a plurality of detectors that are spaced apart from one another on the helmet in determining parameters associated with the helmet. Further difficulties are introduced based on assumptions that the rotating magnet is fixed to the airframe of an aircraft in a known orientation and position and that the helmet position is at least approximately known with respect to the airframe and, therefore, with respect to the rotating magnet. Such constraints may not be acceptable when the rotating magnet concept is applied in more general tracking and locating applications.
The present invention serves to resolve the foregoing limitations and constraints while providing still further advantages.